The present application relates to a positive electrode which is favorably used for, for example, a lithium ion secondary battery, and the like and to a lithium ion secondary battery to which such a positive electrode is applied.
In recent years, following diffusion of portable information electronic devices such as mobile phones, video cameras and laptop personal computers, it is rapidly advanced to realize high performance, downsizing and weight saving of these devices.
As a power source to be used for these devices, disposable primary batteries and repeatedly usable secondary batteries are used. From the viewpoint of favorable comprehensive balance among economy, performance, downsizing, weight saving and the like, among these secondary batteries, lithium ion secondary batteries have been increasingly demanded.
In particular, in recent years, in order to realize higher performance of portable information electronic devices, not only high energy density of a lithium ion secondary battery but also enhancement of cycle properties is required.
First of all, as to the high energy density of a lithium ion secondary battery, it is one of effective methods to use a positive electrode having a high discharge capacity per unit volume. In order to realize such a positive electrode, it is known that (a) selection of an active material and (b) an increase of charge upper limit voltage are important. In recent years, studies for attaining high energy by increasing a charge upper limit voltage are eagerly made.
As a positive electrode active material of lithium ion secondary battery, in addition to LiCoO2, etc., LiNiO2, LiMn2O4 and the like are known.
Here, since LiNiO2 has a relatively high capacity of about 190 mAhg−1, it is necessary to decrease a discharge cutoff voltage for the purpose of obtaining the capacity. However, it may be said that since LiNiO2 is low in average voltage, it is unsuitable for applications requiring a high electric power such as an application to a laptop personal computer. Also, LiMn2O4 is low in capacity so that it is unsuitable for attaining a high energy density of lithium ion secondary battery.
For these reasons, as to a high charge voltage lithium ion secondary battery having an application to a laptop personal computer, it may be said that LiCoO2 which has a high average discharge voltage is especially desirable among the foregoing lithium-containing transition metal oxides.
In a lithium ion secondary battery using LiCoO2 as a positive electrode active material and a carbon material as a negative electrode active material, its charge final voltage is from 4.1 V to 4.2 V. Under such a charge condition, the positive electrode is utilized only in a proportion of from about 50% to 60% relative to the theoretical capacity.
Accordingly, if the charge voltage can be increased, it becomes possible to utilize the capacity of the positive electrode in a proportion of 70% or more relative to the theoretical capacity, and it becomes possible to attain a high capacity and a high energy density of the lithium ion secondary battery.
Actually, for example, as disclosed in WO 03/019731, it is known that by increasing the voltage at the time of charge to 4.30 V or more, a high energy density can be revealed.
On the other hand, as to an enhancement of cycle properties, in a lithium ion secondary battery, a material obtained by coating a positive electrode mixture composed of a positive electrode active material (for example, lithium-containing transition metal composite oxides, etc.), a binder (for example, fluorocarbon resins, etc.), a conductive agent and the like on an aluminum foil as a collector is used.
However, the present inventors set up the charge voltage of an existing lithium ion secondary battery working at 4.2 V at maximum so as to exceed 4.20 V. As a result, it has become clear that there is a problem inherent to the battery of such a system that the discharge amount which can be extracted per cycle is lowered.
As causes of this, there may be considered plural factors including an increase of electron transfer resistance due to a lowering of the contact area of the active material, conductive agent and collector, modification of an electrolytic solution and an increase of diffusion resistance due to an increase of the surface coating film. Among these factors, as to the increase of electron transfer resistance due to a lowering of the contact area of the active material, conductive agent and collector, the matter that the adhesion of the positive electrode mixture in a highly oxidative atmosphere is lowered by increasing an upper limit voltage of charge may be considered to be one of the factors.
Actually, in a battery using a PVDF (polyvinylidene fluoride) binder which is a fluorocarbon resin, a charge-discharge cycle was carried out at an upper limit voltage of 4.2 V and at a charge voltage higher than the upper limit voltage of 4.2 V; after the charge-discharge cycle, the battery was taken apart; and the positive electrode was taken out. As a result, it was confirmed that in the case of carrying out the charge-discharge cycle at a charge voltage higher than 4.2 V, peeling between the positive electrode mixture and the collector was remarkable.
As described above, in the PVDF binder which is a fluorocarbon resin, it has become clear that the adhesion of the positive electrode mixture is lowered and that in case of carrying out a charge-discharge cycle at a charge voltage higher than the upper limit voltage of 4.2 V, the cycle properties are noticeably deteriorated.
Then, for the purpose of improving resistance to peeling of the positive electrode mixture and the collector, the present inventors paid attention to a polyacrylonitrile resin from which a higher adhesive force is obtainable and found that the cycle properties at a high temperature tend to be enhanced by using such a highly adhesive binder.
An example using polyacrylonitrile as a binder for electrode is found in JP-A-2006-40800, and it is disclosed therein that an electrode having excellent pliability and flexibility can be thus manufactured.